The goal of the proposed research is to extend our understanding of psychoactive drugs, particularly those subject to abuse by man, in terms of their effects upon operant behavior and the pharmacologic mechanisms by which these effects are produced. Major emphasis in the proposed investigation is upon the stimulus properties of drugs. It is already known that those classes of drugs which are subject to abuse by man, including barbiturates, ethanol, amphetamines, narcotic analgesics, and hallucinogens, can function as discriminative stimuli. Pharmacologic and behavioral analysis of the stimulus properties of psychoactive drugs is expected to provide a new perspective on these agents as well as to offer the possibility of pre-clinical identification of drugs likely to be abused by humans.